Gas sensor calibration system

ABSTRACT

An apparatus is described for calibrating at least one sensor within a gas detector ( 22 ), which detector has a gas inlet in fluid communication with the or each sensor, the apparatus comprising a housing ( 10 ) that contains: a) a surface ( 24 ) for abutting against the detector ( 22 ); b) a holder ( 14 ) for holding the gas detector ( 22 ) with respect to the housing ( 10 ) in such a manner that a region of the detector containing the gas inlet abuts against the surface ( 24 ) of the housing to form a sealed gas interface between the surface ( 24 ) and the detector ( 22 ); c) a connector for connecting a source of pressurised calibration gas ( 12 ) to the apparatus, d) a conduit ( 32 ) for delivering a calibration gas from the connector ( 34 ) to the interface between the detector and the apparatus housing, e) electrical connections ( 40 ) within the holder for forming electrical connections between the apparatus and a detector held within the holder, and e) a flow controller ( 34, 36 ) within the conduit for providing calibration gas at a predetermined level to the interface, the flow controller including an electrically-operated valve ( 36 ) being controllable for initiating and terminating the flow of calibration gas through the conduit ( 32 ) by means of signals received from the detector via the electrical connections ( 40 ). The whole calibration is controlled by the detector and not the calibration apparatus.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage application ofInternational Application PCT/GB01/05472, filed Dec. 11, 2001, whichinternational application was published on Jun. 20, 2002 asInternational Publication WO 02/48705. The International Applicationclaims priority of British Patent Application 0030167.1, filed Dec. 11,2000.

TECHNICAL FIELD

The present invention relates to a system for calibrating gas sensors,which are used in gas detection instruments and gas analysers (the term“detector” will be used in this specification to cover both types ofapparatus) to detect or analyse potentially hazardous environments toensure that the sensors provide accurate readings.

BACKGROUND ART

Portable gas detectors containing electrochemical gas sensors arewell-known for monitoring potentially hazardous environments, forexample mines, tunnels, sewers and other closed environments. Suchdetectors are generally of the type in which gas from the atmospherecomes into contact with the sensors(s) within the detector by diffusion.Electronic circuits within the detector convert the output signal fromeach sensor into a reading of the amount of gas detected. The sensoroutput per unit amount of gas can vary with time and hence periodiccalibration is required to ensure that the detector reading is accurate.Safety regulations require that the sensors within the detector aretested on each occasion that they are taken into a potentially hazardousenvironment and calibrated according to manufacturer's recommendationsand that is indeed good commercial practice but it is frequently notcomplied with for reasons of cost and time.

Currently, sensors within such detectors are calibrated by passing acalibration gas of known fixed composition from a compressed as bottleat a predetermined flow rate through a loose pipeline to a hood clippedonto the detector. The calibration gas entering the detector displacesambient air within the detector so that the environment that the sensoris exposed to is composed wholly of calibration gas. Excess calibrationgas flows out of the hood and is vented to atmosphere and so theprocedure is wasteful of calibration gas, which is expensive. Inaddition, the gas required for calibration could be hazardous and ifsubstantial quantities are vented, calibration should be carried out ina controlled environment. Typically a high flow rate of about 0.5litres/minute are used since a lower rate is prone to error resultingfrom draughts and incorrect setting of the valves controlling the flowof gas.

The calibration gas is allowed to flow until the sensor output hasreached a steady state. Since the calibration gas has a knowncomposition, the gain of the circuits within the detector that convertthe output signal from each sensor into a reading of the amount of gasdetected can be adjusted to provide the correct reading.

The known calibration procedures are not straightforward and the correctsetting of the valves to achieve the correct gas flow rates and theadjustment of the settings in the detectors is a skilled job requiringtraining and so calibration has hitherto been performed onlyperiodically, typically every 3 to 6 months by sending the detectors tothe original manufacturer or appointed service agent. This requires astock of spare detectors to be held, or an expensive site visit toperform the calibration. For these reasons calibration has beenexpensive and consequently is often not performed as frequently as theregulations require.

U.S. Pat. No. 4,854,153 describes an automatic gas sensor calibrationapparatus that exposes the sensor to two different concentrations of gasto perform the calibration. If a fault is detected in the gas supply,calibration is prematurely terminated to save calibration time. Thecalibration apparatus totally controls the calibration measurementsaccording to a regime that is pre-set by the apparatus and thecalibration values measured are stored within a memory in the apparatus.

U.S. Pat. No. 5,655,894 describes a gas sensor calibration systemwherein gas is drawn into the calibration system by a pump, where it ismetered out by a piston-cylinder arrangement.

The present invention provides an alternative, quicker and more costeffective method of calibrating gas sensors that can be performedquickly on site with minimal training. This makes it practically andeconomically feasible for the personnel entering a hazardous environmentto perform a calibration on each occasion that they enter such anenvironment, thereby increasing safety.

DISCLOSURE OF INVENTION

According to the present invention, there is provided an apparatus forcalibrating at least one sensor within a gas detector, which detectorhas a gas inlet in fluid communication with the or each sensor, theapparatus comprising a housing that contains:

-   -   a) a surface for abutting against a detector;    -   b) a holder for holding a gas detector with respect to the        housing in such a manner that a region of the detector        containing the gas inlet abuts against the surface of the        housing to form a sealed gas interface between the surface and        the detector;    -   c) a connector for connecting a source of pressurised        calibration gas to the apparatus,    -   d) a conduit for delivering a calibration gas from the connector        to the interface between the detector and the apparatus housing,    -   e) electrical connectors within the holder for forming        electrical connections between the apparatus and a detector held        within the holder, and    -   f) a flow controller within the conduit for providing        calibration gas at a predetermined level to the interface, the        flow controller including an electrically-operated valve being        controllable for initiating and terminating the flow of        calibration gas through the conduit by means of signals received        from the detector via the electrical connections.

Because all the components necessary to perform calibration are allsupplied within a single housing, the distance between the pressurisedcalibration gas connector and the surface for abutting against thedetector can be kept to a minimum, e.g. less than 10 cms, morepreferably less than 5 cms, so that the amount of gas space within theapparatus that must be flushed with calibration gas is kept to a minimumto save calibration gas and to speed up calibration.

The detector preferably includes a calibration circuit for calibratingautomatically the output of the detector to accord with the compositionof the calibration gas.

The apparatus and the detector each includes electrical connectors forforming electrical connections between the detector and the apparatuswhereby the operation of the calibration apparatus, e.g. the flow ofcalibration gas to the detector, can be controlled in accordance withinstructions held within the detector. To that end, the apparatusincludes an electronically controllable valve for initiating andterminating the flow of calibration gas through the conduit inaccordance with signals received from the detector. In this way thecalibration can be performed automatically with sufficient calibrationgas being supplied for the signal from the sensor(s) within the detectorto reach a steady state. Since calibration is wholly under the controlof the detector, there is no need for specialised staff (or indeed anystaff) to perform calibration.

The surface against which the detector abuts is preferably surrounded bya compliant seal to form a gas-impervious seal around the interfacebetween the detector and the housing.

A detector may be pressed against the surface of the housing by a springbiased arm, or some other mechanical arrangement that urges the detectoragainst the calibration apparatus.

The present invention also provides a method of calibrating at least onesensor within a gas detector that has a gas inlet in fluid communicationwith the or each sensor, the method using an apparatus comprising ahousing that contains:

-   -   a) a surface for abutting against a detector to form a sealed        gas interface between the surface and the detector,    -   b) a source of pressurised calibration gas containing a known        concentration of gases that the at least one sensor is        responsive to, and    -   c) a conduit for delivering the calibration gas to the interface        between the detector and the housing,        the method comprising:    -   i urging a gas detector against the surface of the housing such        that the region of the detector containing the gas inlet abuts        against the surface of the housing    -   ii allowing calibration gas to flow from the source to the        sealed gas interface at a predetermined rate, and    -   iii calibrating the at least one sensor within the detector such        that the detector provides a reading corresponding to the known        concentration of gases within the calibration gas,    -   wherein the detector initiates the flow of calibration gas to        the sealed gas interface, automatically calibrates the at least        one sensor within the detector and stops the flow of calibration        gas following calibration.

According to this method, the detector may initiate the flow ofcalibration gas, automatically calibrate the at least one sensor withinthe detector and stop the flow of calibration gas following calibration.

The detector advantageously generates an error signal if the calibrationprocess is not completed within a predetermined time, e.g. 1 minute, orif the signal from the at least one sensor during calibration fallsoutside a predetermined range.

The present invention also provides a detector.

By reducing the volume of gas space between the source of pressurisedcalibration gas and the detector and by forming a gas tight seal betweenthe calibration apparatus and a detector, the predetermined flow rate ofcalibration gas can be as low as 0.1 litre/minute ±20% and permits thesensor(s) to come to an equilibrium value quickly which reduces theconsumption of expensive calibration gas. Also, because the detector canstop the flow of calibration gas immediately it detects the sensoroutput(s) have reached a steady state, less calibration gas is requiredfor each calibration.

By including the connection to the gas cylinder and the outlet to thesensor within a single housing of the calibrating apparatus, it ispossible to reduce the length of the gas path between the source ofcalibration gas and the sensor itself, which in turn means that thesensor calibration can be done more quickly than hitherto and becausethe calibration is controlled by the detector, there is no need forexpensive personnel to perform calibration.

BRIEF DESCRIPTION OF THE DRAWINGS

A calibration apparatus according to the present invention will now bedescribed by way of example with reference to the drawings in which:

FIG. 1 is a perspective view of a calibration apparatus according to thepresent invention;

FIG. 2 is a perspective view of the calibration apparatus of FIG. 1 is adifferent configuration;

FIG. 3 is a perspective view of the calibration apparatus of FIGS. 1 and2 together with a detector;

FIG. 4 is a cross sectional view through the apparatus in theconfiguration shown in FIG. 2.

FIG. 5 is a schematic circuit diagram showing the connections betweenthe calibration apparatus and a sensor being calibrated; and

FIG. 6 is a logic diagram showing the calibration process using theapparatus of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring initially to FIGS. 1 to 3, the apparatus comprises a housing10 containing a cylinder 12 of pressurised calibration gas that holds amixture of that a detector is or may be sensitive to, for exampleoxygen, carbon monoxide, flammable gases and hydrogen sulphide, in aninert carrier, e.g. nitrogen. The gases are present in knownpredetermined concentrations. The housing includes an end panel 14 thatcan be slid out (see FIG. 2) while still being attached to the mainhousing 10 by means of an arm 16. As will be more fully described below,the arm 16 is spring loaded by spring 20 to urge the end panel 14towards the main housing. However, it can be latched in place by meansof a latching mechanism that can be released by operating a latch 18.Thus, when the end panel 14 is pulled out, it is held in the openposition shown in FIG. 2. However, when the latch 18 is operated, thelatching mechanism is released and the end panel is pulled by a spring20 (see FIG. 4) towards the main housing body.

A detector 22 can be placed within the space between the main housingbody 10 and the end panel 14. The detector includes a face (not shown)that contains an inlet (not shown) that, in normal detecting operation,allows gas from the atmosphere being monitored to reach the sensorswithin the detector 22 by diffusion. When the latch 18 is released, itis urged by the end panel 14 and the spring 20 towards the main housing10; the end face 24 of the main housing (against which the detector isurged) is surrounded by a compliant seal 26 so that the face of thedetector that contains the gas diffusion inlet (not shown) is sealedagainst the end face 24 of the main housing in a gas-tight manner. Theend face 24 includes gas inlet port 28 and a gas outflow port 30 thatwill be described in greater detail in connection with FIG. 4.

Referring now to FIG. 4, the gas cylinder 12 is shown connected by aknown fitting to a conduit 32 containing a pressure/flow regulator 34that produces a constant flow of calibration gas, e.g. at a rate of 0.1litres per minute. The conduit also contains a solenoid valve 36 thatopens and closes the conduit in response to an electrical signalreceived from a microprocessor 21 (see FIG. 5) within the detector. Theconduit 32 ends in inlet port 28 described above.

In the course of calibration, gas already within the detector, e.g. air,is flushed out by the calibration gas, which passes through outlet port30. An exhaust conduit 38 vents such gas to atmosphere or to a safedisposal arrangement.

The arm 16 also contains electrical connectors 40 (only one shown) thatengage with corresponding connectors (not shown) within the detector 22.The detector 22 includes a microprocessor 21 (see FIG. 5) that controlsthe calibration performed by the calibration apparatus. The signals fromthe detector 22 are fed via connectors 40 to open and close the solenoidvalve 36. A microswitch 42 is also provided having an rocker 41 thatcloses the switch contacts when pressed upwardly by a land (not visible)on the arm 16. The land is positioned on the arm such that it pressesagainst the rocker 41 (and hence closes the microswitch) when thedistance between the end panel 14 and the face 24 corresponds to thewidth of the detector 22. This means that the microswitch is closed whenthe detector is in place and pressed against end face 24 but otherwisethe microswitch is open. Thus the microswitch can detect that a detectorhas been installed correctly within the calibration apparatus.

When the detector 22 is placed within the space 44 between the end panel14 and end face 24 of the main housing, the latch 18 is released,thereby allowing the spring 20 to urge the end panel 14 in the left handdirect (as seen in FIG. 4), thereby pressing the detector 22 against theend face 24 of the main housing. The gas inlet of the detector (notshown), which normally allows gas from the atmosphere to diffuse intothe detector to reach sensors within the detector, is sealed against theend face 24 so that a gas tight seal is formed around the detector gasinlet of the detector and the end face 24. The ports 28 and 30 are thusin fluid communication with the gas inlet of the detector. Gas suppliedalong the conduit 32 can thus pass into the inlet of the detector andreach the sensors within the detector. Likewise, gas flushed from thedetector can be vented via port 30 to the atmosphere.

FIG. 5 shows the connections between, on the one hand, the microswitch42 and the solenoid valve 36 and, on the other hand, the microprocessor24 within the detector 22. When a detector has been installed within thespace 44 between end panel 14 and end face 24, the microswitch is, asdescribed above, closed which causes a positive voltage V from rail 23to be applied via contacts 40 to the microprocessor, thereby indicatingthat a sensor has been properly installed within the space 44 and thatthe arm 16 has been retracted. The microprocessor can then pass controlsignals via contacts 40 to the solenoid valve 36 and take control of thecalibration process. However, the user is first asked on a screen (notshown) whether he wishes a calibration cycle to be initiated. Heinitiates the calibration cycle by pressing a push button 46 on the mainhousing. Once the switch 46 is activated, the complete calibrationprocedure is taken over by the microprocessor 21 within the detector 22.

The calibration procedure is thus as follows (referring to FIG. 6):

-   1 The end panel 14 is pulled away from the main housing 10 and    latched in an open position (Box 1).-   2. The detector 22 is placed in the space 44 and the latch is    released to urge the end panel 14 towards the main housing and hence    to urge the detector against the face 24 (Box 2);-   3. The microswitch 44 is closed by the land, indicating that the    detector is correctly installed (Box 3); if not, an error signal is    reported (Box 4)-   4. Once the user has approved calibration by pressing push-button    46, the microprocessor 21 sends a signal via contacts 40 to the    solenoid valve 36 to open the solenoid valve 36 thereby allowing    calibration gas to flow from the gas cylinder 12 through the flow    control valve 34, at a flow rate of approximately 0.1 litres per    minute, through conduit 32 and out through port 28 into the inlet of    the detector 22 (Box 5).-   5. A timer within the microprocessor is started when calibration is    initiated; (Box 6)-   6. The sensors within the detector 22 can therefore register and    respond to the gas supplied. The calibration gas contains a known    fixed concentration of various gases to be detected, e.g. oxygen,    carbon monoxide, hydrogen sulphide and a flammable gas, e.g. butane.    Gas flushed out from within the detector 22 can escape via port 30    and conduit 38 to the atmosphere. It generally takes approximately    30 seconds to reach a steady state reading.-   7. The microprocessor “reads” the signals from the sensors (Box 7).    The microprocessor then performs a loop (Boxes 7, 8 and 9); if the    loop is being performed for the first time or if the signal from a    sensor is not the same (within predefined tolerances) as the signal    on the previous iteration of the loop (Box 8), the timer is    interrogated (Box 9). If the time elapsed since the initiation of    the calibration is less than 60 seconds, the signal from the sensors    is again read (Box 7). The loop is repeated until the signals from    the sensors have reached a stable steady state reading or 60 seconds    have elapsed.-   8. If more than 60 seconds have elapsed and steady state readings    from the sensors have not been detected, an error signal is    generated and the calibration fails (Box 10);-   9. If steady state readings from the sensors have not been detected    within 60 seconds, the microprocessor interrogates the magnitude of    the signals from the sensors (Box 11); if they fall outside    predetermined ranges, an error signal is generated and the    calibration fails (Box 10);-   10. If the calibration has not failed, the microprocessor within the    detector calibrates the sensors by adjusting the gain of the    detector to produce a reading exactly corresponding to the known    composition of the calibration gas (Box 12)-   11. The detector 22 is then removed from the calibration apparatus    (Box 13).

The error signal (Box 10) could be caused either by a malfunction of thesensor (indicating that it needs replacing) or by dirty filters withinthe detector 22. Thus, if an error signal is generated, the filtershould first of all be cleaned or replaced and calibration re-initiated.If the detector, on recalibration, also fails, then this is indicativethat one or more of the sensors should be replaced. If, afterreplacement of the sensors, the detector still generates an errorsignal, then that is indicative of a fault in the detector itself.

The faster the flow rate of calibration gas to the detector, the fasterit reaches the steady state reading. Furthermore, at low flow rates ofthe calibration gas, the magnitude of the final steady state signal froma sensor will depend on the flow rate. However at higher flow rates(approximately 0.3 to 0.5 litres per minute or greater), the finalsteady state signal from a sensor will be largely independent of theflow rate. The prior art has generally used flow rates of 0.5litres/minute and avoided using flow rates as low as 0.1 litre perminute since, if the rate of flow of calibration gas were to vary at alow flow rate, the magnitude of the steady state response signal couldvary and hence would be unreliable. On the other hand, it is desirableto use as low a flow rate of gas as possible, firstly because it moreclosely approximates to the normal operation of a gas detector, wherebygas diffuses into the detector rather than being pumped into thedetector, and secondly, the lower the rate of flow, the less calibrationgas is used. We have found that, by reducing the length of the conduit32 and by clamping the detector against the end face of the housing 10,and by providing the whole calibration equipment within a singlehousing, reliable readings can be obtained for calibration of a gassensor, even at 0.1 litres per minute.

It can be seen that calibration can be completed simply and withoutspecial staff training within approximately one minute and can beundertaken by personnel prior to entering a hazardous area. Thus, thecalibration can replace the inexact “testing” of the detector, whichmerely shows that the sensors are operating rather than that theyprovide accurate readings. There is no need to send the detector awayfor calibration or arrange for special visits by trained calibrationstaff. Also, the arrangement of the calibration apparatus allows the usefor low flow rates of calibration gas without being affected bydraughts. Accordingly, the present invention provides a cheaper andsafer system for testing and calibrating gas detectors prior to enteringhazardous area.

Because the whole calibration process is controlled by the detector andnot the calibration apparatus, the calibration data is held within themicroprocessor and so the calibration data for each sensor is storedwithin the detector itself and so for example the improper functioningof a sensor can be detected and a signal generated that the sensorshould be replaced. In addition, by controlling the calibrationapparatus from the detector, the calibration apparatus will berelatively simple and cheap to manufacture. The detector will generallyalready have a microprocessor for its normal operation and so theincorporation of the software for controlling calibration and forlogging calibration data into the microprocessor does not make thedetector any more expensive.

The apparatus may also be made light enough for it to be portable andcompact enough that it can be easily stored. Thus it can be used readilyin the most convenient position for calibration.

1. An apparatus for calibrating at least one sensor within a portable gas detector, which detector has a gas inlet in fluid communication with the at least one sensor, the apparatus comprising a housing that contains: a) a surface for abutting against a gas detector; b) a holder for holding the gas detector with respect to the housing in such a manner that a region of the detector containing the gas inlet abuts against the surface of the housing to form a sealed gas intertace between the surface and the detector; c) a connector for connecting a source of pressurized calibration gas to the apparatus, d) a conduit for delivering calibration gas from the connector to the interface between the detector and the apparatus housing, e) electrical connectors within the holder for forming electrical connections between the apparatus and the detector held within the holder, and f) a flow controller within the conduit for providing calibration gas at a predetermined level to the interface, the flow controller including an electrically-operated flow control valve which is controllable for initiating and terminating a flow of calibration gas through the conduit by valve control signals received by the valve from the detector via the electrical connections.
 2. An apparatus as in claim 1 wherein the distance between the connector and the surface for abutting against a detector is less than 10 cms.
 3. An apparatus as in claim 2, wherein the distance is less than 5 cms.
 4. An apparatus as in claim 1 wherein the flow control valve supplies calibration gas to the interface at a constant flow rate.
 5. An apparatus as in claim 1 wherein the surface for abutting against a detector is surrounded by a compliant seal.
 6. An apparatus as in claim 1 wherein the holder comprises a spring-biased arm for holding the detector and urging the detector against the surface.
 7. An apparatus as in claim 1, which includes a source of calibration gas.
 8. An apparatus as in claim 1 which includes a microprocessor to transmit valve control signals to the flow control valve via the electrical connections to control the opening and closing of the flow control valve.
 9. An apparatus as as in claim 8, wherein the detector comprises at least one electrochemical gas sensor.
 10. An apparatus as in claim 8 wherein the detector includes a calibration circuit for calibrating an output of the detector.
 11. An apparatus as in claim 9 which includes a memory for storing calibration data.
 12. An apparatus as in claim 2, wherein the flow control valve supplies the gas to the interface at a constant flow rate.
 13. An apparatus as in claim 9 wherein the detector includes a calibration circuit for calibrating an output of the detector.
 14. An apparatus as in claim 10 that includes a memory for storing calibration data. 